It was the then owner and editor of this newspaper who, a hundred years ago this summer, put the proposition as succinctly as it has ever been put. Now that Labour and the Liberals had abandoned their electoral pact and decided to fight each other in every parliamentary seat, wrote CP Scott in July 1912, "it is quite possible that while Liberalism and Labour are snapping and snarling at each other, the Conservative dog may run away with the bone".

We should follow Scott's essay a little further. Should Labour and the Liberals be regarded as two divisions of "the party of progress", he then asked? Or were they in "direct and necessary antagonism"? Few Liberals would hesitate as to the answer, Scott answered.

"Towards such a party," he concluded, "the natural attitude of Liberals would seem to be not one of jealousy or hostility but of frank and intimate co-operation." In the absence of proportional representation, Scott wrote (yes, there were even Liberal calls for electoral reform a hundred years ago), "there is nothing for it but mutual consideration, a fair regard for each other's numerical claims, respect for the real wishes of the progressive elements in constituencies – in a word, compromise".

A century on, Scott's powerful plea for co-operation between the Liberal and Labour traditions is proof both of his insight and of his ineffectiveness. On the one hand, his words remain an almost spookily relevant depiction of the broken condition of British progressive party politics today. On the other, it is obvious that, after a century of intermittent effort, the supporters of Lib-Labbery have got precisely nowhere. Under the Conservative-Lib Dem coalition, they naturally appear further apart than ever.

As the Lib Dem conference closes and as Labour prepares to take the stage in Manchester this weekend, there is a lot of gossip about the possibility of a Lib-Lab coalition in a new hung parliament after 2015. And shards of significant action too. At Brighton, Vince Cable flirted publicly with Ed Miliband and left his door open for Lib-Lab co-operation in the next parliament. On the fringe, Jon Cruddas and Andrew Adonis, two of the more open-minded of Labour's leaders, shared a platform to discuss pluralism with Ming Campbell and Jo Swinson. If Nick Clegg were to fall on his sword before the next election and be replaced by Cable – both very large "ifs" – the speculation about closer relations would become deafening. Even if Clegg remained as leader, the possibility could not be altogether ruled out.

But would all the talk be mother to the actual deed? Many people assume so, if the numbers allow. Given the nature of the problems facing countries like Britain, given the enduring intellectual overlap between the two parties, given the continuing collapse of 20th-century party loyalties, and given the possibility of a parliament next time in which Labour is the largest single party and the Lib Dems survive in rather larger numbers than the polls currently imply, it looks like a no-brainer. Surely this is the time – better late than never from a Manchester Guardian perspective – for the split between Labourism and Liberalism to be healed and for the progressive sword that was broken a century ago to be reforged by some latterday social democratic Siegfried.

Don't be too sure. Even those of us who are instinctively supportive of Lib-Lab convergence and remain believers in that much-abused concept the progressive majority (though there are less progressive majorities too) need to be realistic. We may be strongly in favour of a common project between the two traditions and parties, but we should also be deeply conscious of the many failures that litter the history of attempts to achieve such a goal.

Those failures stretch from the end-of-the-Edwardian-era pact that triggered Scott's essay, through the collapse of the MacDonald governments in the interwar period and the failure of the SDP in the 1980s, to the acrimony that surrounded the failure to make the numbers and the politics add up for a Lib-Lab deal after the last election in May 2010. Such acrimony is now, if anything, even stronger in many parts of both parties.

Two novel sources this week particularly reinforce this caution about assuming that another hung parliament would automatically see a Lib-Lab coalition. The first is a visit to James Graham's National Theatre play, This House – a political anorak's dream night out that makes a compelling piece of theatre out of the 1974-79 Labour government's attempts to cling to office – including a brief Lib-Lab pact – as Thatcherism looms off-stage. It is a big reminder that the default Labour tradition in past hung parliaments is not to form a coalition but to govern as a minority. Past Labour governments in such circumstances have foolishly preferred making compromises with leftwing backbenchers than sharing power with Liberals. And that could happen again.

The other new source is Jack Straw's memoirs, Last Man Standing, in which the modern era's most accomplished Labour pragmatist makes clear that, even if the numbers had permitted it in 2010, "it wasn't my bag" to work alongside a party which he simply did not trust and with which he did not feel emotionally at ease – and this although Straw has plenty of other friends outside the Labour tribe with whom he is comfortable. If even a Labour reformer and moderniser, which Straw generally is – his book contains an imaginative proposal for putting the prime minister and cabinet under statutory authority – would get into bed with the Lib Dems only under extreme duress, then what of those in the party who might prefer to make a gesture of principle out of a refusal to govern with the Lib Dems.

A Lib-Lab coalition does not have to be either party's principal aim. Yet if such a coalition has to be formed, it is better that it is a good one rather than a bad one. That requires advance trust-building between key individuals. It requires some advance thought about what kind of programme such a coalition might realistically adopt. It also, learning from the Cameron-Clegg experience, requires looking around corners to put in place some of the processes and protocols that would apply. How far either party really wants to sanction such work is not clear – a signal from Ed Miliband would be extremely helpful. But it is work that should be done – and done now – on both sides. If Labour is serious, it occurs to me that Jack Straw might be an ideal person to help do it.

• This article was amended on 27 September 2012. The original subheading used the word "principle" where its homophone "principal" was meant. This has now been corrected